KIMBLE, M.*; COURSEY, Y.; AHMAD, N.; HINSCH, G.W.: Behavior of the yolk nuclei during embryogenesis, and development of the midgut diverticulum in the horseshoe crab, Limulus polyphemus.

Horseshoe crabs have existed for hundreds of millions of years, and have been the subject of study for decades, however there are still some significant gaps in our understanding of these fascinating animals. Studies of horseshoe crab development conducted in the late 1800’s and early 1900’s provided extensive detail regarding gastrulation, tissue formation and development of most of the major organ systems. However, relatively little detail is available on the earliest stages of development and the contribution of the yolk cells/nuclei to development. This deficiency is due in large part to the difficulty of preparing the eggs/embryos for sectioning. Kingsley (1892) stated that the horseshoe crab eggs were the most refractory objects he had ever seen, and that the support media available at the time were unable to penetrate the dense yolk. Using histological resins developed in recent years we have overcome these difficulties, and are now able to begin filling in the gaps in our knowledge of horseshoe crab development. We show here that the yolk nuclei undergo a series of changes during embryogenesis, prior to cellularizing and forming the midgut epithelium in the first instar larva. We also show that formation of the digestive diverticulum is a two step process in which a mesodermally derived lamina divides the yolk mass into distinct lobes, defining the boundaries of the digestive caecae. Once the boundaries of the caecae are established the yolk nuclei cellularize to form the endoderm epithelium.